NOUN (3)

1. a negative statement; a statement that is a refusal or denial of some other statement;

2. the speech act of negating;

3. (logic) a proposition that is true if and only if another proposition is false;

The Collaborative International Dictionary of English v.0.48:

Negation \Ne*ga"tion\, n. [L. negatio, fr. negare to say no, to
   deny; ne not + the root of aio I say; cf. Gr. ?, Skr. ah to
   say; cf. F. n['e]gation. See No, adv., and cf. Adage,
   Deny, Renegade.]
   1. The act of denying; assertion of the nonreality or
      untruthfulness of anything; declaration that something is
      not, or has not been, or will not be; denial; -- the
      opposite of affirmation.
      [1913 Webster]

            Our assertions and negations should be yea and nay.
                                                  --Rogers.
      [1913 Webster]

   2. (Logic) Description or definition by denial, exclusion, or
      exception; statement of what a thing is not, or has not,
      from which may be inferred what it is or has.
      [1913 Webster]

WordNet (r) 3.0 (2006):

negation
    n 1: a negative statement; a statement that is a refusal or
         denial of some other statement
    2: the speech act of negating
    3: (logic) a proposition that is true if and only if another
       proposition is false